Site suitability assessment of spotted gum (Corymbia citriodora subspecies Variegata) forest plantation in south east Queensland for carbon sequestration

Site suitability assessment of spotted gum (Corymbia citriodora subspecies Variegata) forest plantation in south east Queensland for carbon sequestration

The inevitable development brought about by human intervention in the global natural ecosystem has led to a continuous increase in greenhouse gas (GHG) emissions into the atmosphere (IPCC 2013), thereby warming the earth. Forest and other vegetation landscapes have the potential to mitigate this warming by serving as the sinks or storages of carbon dioxide. However, land suitability for carbon sink enterprises must be identified to effect a more productive, profitable and efficient sequestration. Additionally, the impact of soil salinity in the suitability of potential forestation sites and carbon sequestration capacity of forest plantation in saline affected areas is not well understood.

This research addressed the problem of site suitability by applying the most appropriate process based model particularly the 3-PG (Physiological Principle in Predicting Growth) to enhance the accuracy of estimated biomass for spotted gum. Salinity was incorporated through the mortality impact of varying salt concentrations in the spotted gum. As carbon sequestration endeavour via forestation was deemed risky in marginal areas such as those with salinity problems, the net present value (NPV) of spotted gum forest plantation was incorporated to determine its financial benefits. Site suitability of this forestation endeavour in southeast Queensland was determined and visualised using the geographical information system (GIS).

Mathematical models for spotted gum mortality and soil salinity were developed and integrated with the 3-PG model. Parameterisation of the model using specific climatic and bio-physical parameters for spotted gum was conducted prior to the simulation. These provided confidence in the biomass simulation and projection of carbon sequestered until the end of the rotation period. The estimated total biomass (aboveground and belowground biomass) were converted to carbon and tabulated. The tabulated results were converted into maps using GIS techniques and the Net Present Value of spotted gum was calculated based from its potential for timber, carbon and salinity amelioration. The utilisation of spatial mapping tools, specifically GIS, generated potential suitable sites for carbon sequestration activities in the SEQ region.

The study generated maps to identify potential locations suitable for Carbon Farming Initiative (CFI) eligible carbon sequestration projects. The site suitability index (SSI) map showed suitable sites in the northern part of the study area located at SEQ 1 and in the southern part at SEQ 2 and SEQ 8. The SSI map also indicated locations for potential investment in forestation projects. It also suggested that success of the carbon sequestration activities cannot be guaranteed in high rainfall areas where salinity could pose a challenge.

If spotted gum plantations are established in southeast Queensland and only the conventional timber is accounted as the source of revenue, then the financial benefits are limited to high rainfall areas with a mean annual increment (MAI) of more than 18 m3 ha-1 yr-1. However, under high saline conditions the viability is questionable. When established under high saline affected areas with carbon and salt amelioration incorporated, then carbon sequestration may become profitable.

However, this may only be applicable in a scenario where the carbon price was increased and a conventional timber is added with carbon and soil amelioration
benefits. Though suitable sites are limited where this activity is profitable, the potential of spotted gum for soil amelioration under saline affected areas is significant even for an extended long period of time.